The challenges of obtaining valid Blowout Preventer (BOP) pressure tests in an efficient manner have increased due to greater water depths, deeper drilling horizons, and higher test pressures. FIG. 1 shows the important components involved in testing a subsea BOP stack. A drill string tool or test plug is lowered into the interior or throughbore of the BOP and it seats at the lower end of the BOP to seal off the well components further down the wellbore. The system is a pressure vessel comprised of the test line 10 from the Cementing Unit (CU) 12 and the drillpipe 14 from the 13 surface of the rig 16 down to the BOP stack 18 at the mudline 20. In this work, the capacity of the BOP pressure vessel is referred to as the “test volume.” A choke line 24 and a kill line 26 connect the throughbore at the interior of the BOP to the CU 12. The valves (e.g., annular preventers, pipe rams, shear rams, etc.) 22 in the BOP stack are tested in sequence by closing each valve and then pumping fluid from the CU into the test volume until a “target pressure” is reached (the “pumping phase”). At the target pressure, pumping stops and the test volume is closed until a test is deemed valid (the “shut-in phase”). In deepwater wells, the duration of the shut-in phase can be as long as 45 minutes when Synthetic Based Muds (SBMs) are used. Pressure testing a BOP with SBM requires lengthy testing times as a result of pressure/volume/temperature (PVT) influences associated with SBM. PVT influences are especially pronounced in deepwater and high-pressure test environments.
In the USA federal regulations state that a test is valid when the required pressure is held steady for 5 minutes (“Oil and Gas Drilling Operation,” Subpart D, 30 CFR Ch. II, Jul. 1, 1999 Edition). Data from a BOP test is historically recorded on a four-hour circular chart recorder shown in FIG. 2. Validation of a test based on the pressure trace on a chart recorder is based on individual judgment. Often, a test is repeated when visual inspection of the chart recorder trace deems it invalid. Frequently, test durations are longer than necessary to help ensure a valid test. The basic chart recorder used on a majority of oil rigs today was patented over one hundred years ago (Wittmer, G. X.: “Recording Apparatus for Fluid Meters,” U.S. Pat. No. 716,973).
The problem of BOP testing has existed for some time. Considerable time and effort is expended each year to perform BOP tests. Validating each individual pressure test requires excessive time as a result of waiting on a declining pressure to stabilize. The time to stabilization on each test can take hours. In spite of this, BOP testing schemes have not progressed. Actually, the problem has become aggravated with the passage of time because each year more and more testing is conducted using time consuming processes.